1. Field of the Invention
The present invention relates to a method for separating a liquid mixture by pervaporation. More particularly, the present invention relates to a method for recovering isopropanol of high purity by removing water from a water-containing isopropanol after being used for cleaning precision parts such as semiconductors.
2. Discussion of Background
As one of methods for separating liquid mixtures, so-called pervaporation is known wherein a certain specific component in a liquid mixture is separated by means of a membrane having a specific affinity with the component by placing the liquid mixture on one side (the primary side) of the membrane and vacuuming the other side (secondary side) by a vacuum pump or purging the secondary side with an inert gas so that the partial vapor pressure of the component passing through the secondary side is maintained to be lower than the equilibrium vapor pressure at the primary side.
This separation method is useful for separating a liquid mixture which can not be separated by a usual distillation method, such as an azeotropic mixture or a mixture of liquids having boiling points close to one another.
On the other hand, as a method for cleaning precision parts such as semiconductor wafers, it has been common in recent years to employ a method wherein such parts are cleaned with water, followed by cleaning with isopropanol (hereinafter referred to simply as IPA). In such a case, it has been attempted to remove water from the water-containing IPA after use to recover and reuse IPA of high purity. As the method for effectively separating water from the water-containing IPA, pervaporation may be considered.
In order to carry out this pervaporation method efficiently, it is advisable to maintain the temperature of the liquid mixture at a high level so that the equilibrium vapor pressure at the primary side will be high. However, the temperature of the liquid tends to decrease due to the latent heat of vaporization of the liquid. It is conceivable to increase the temperature of the liquid to be supplied to the membrane module unit, but such a method has a limitation in view of e.g. the heat resistance of the separating membrane. A method for efficiently supplying the heat corresponding to the latent heat of vaporization to the liquid mixture, is proposed, for example, in Japanese Examined Patent Publication No. 5441/1990 or No. 21288/1990. However, such a method is still inadequate from the viewpoint of the efficiency.
The apparatus for separating IPA and water used for cleaning semiconductor wafers, is desired to be incorporated in the production line for such wafers and accordingly is required to be as compact as possible and capable of treating the cleaning solution as much as possible. However, there has been no apparatus available which is compact and efficient enough to comply with such requirements.